(1) Field of the Invention
The invention relates to a beam for a rotorcraft rotor and particularly to a helicopter rotor.
(2) Description of Related Art
In general, a rotorcraft is an aircraft that differs from other powered aircrafts mainly in their ability to fly both at high cruising speeds and to fly at low speeds or to hover. This ability is obtained by providing the rotorcraft with at least one main rotor having an axis that is substantially vertical.
The main rotor comprises a rotary wing that provides the rotorcraft with lift and possibly also with thrust and control moments. The control of the rotorcraft in flight is facilitated by varying the cyclic and/or collective pitch of the blades of the rotary wing.
Conventionally, a rotorcraft includes in principle a group of engines comprising at least one fuel-burning engine such as a piston engine or a turbine engine. A gearbox connects the group of engines to a hub of the main rotor, a number of rotor blades and a corresponding number of beams connecting each blade with the hub.
During operation, the rotor must on the one hand resist the lift, drag, centrifugal forces (and corresponding moments) of the blades that apply during rotation, and on the other hand must permit the flapping motions, torsion or pitch motions and lead/lag motions of the blades.
The many different systems utilized for this task are variations on basic designs, such as so-called articulated rotors and hingeless rotors. The articulated rotor utilizes rigid beams equipped with hinges and bearings to facilitate the aforementioned blade motions. The bearingless rotor comprises special composite material beams, so-called flexbeams that are flexible enough to bend and twist to allow blade movement without bearings and additional mechanics.
In a bearingless rotor, the function of discrete hinges and bearings is performed by structures which are weak against bending and/or torsion in certain areas or portions of the beam, but which of course transfer all the loads (lift and drag (shear forces), centrifugal forces), bending and twisting moments, and so forth. An equivalent or virtual offset of a flap hinge, lead lag hinge and pitch hinge from the rotation axis of the hub and output shaft is defined for such bearingless rotors.
Hingeless rotors are distinguished from bearingless rotors by a flexible beam to connect each blade to the rotor hub, the connection using conventional pitch bearings (metallic bearings or bearings with composite materials).
The document U.S. Pat. No. 5,286,167 describes a flexbeam helicopter rotor having a connection between the flexbeam and the hub to allow a blade flapping motion only around a flapping axis. The blade flapping motion is obtained at the location of the connection between the flexbeam and the hub. A load decoupling permits the use of a thinner flexbeam spar with the attendant advantage of rotor weight and size reduction, and the additional advantage of a reduced flapping hinge effecting offset, or flapping axis.
The document DE19915085 discloses a rotor blade mounting for a helicopter with a flexbeam between the rotor head and the rotor blade and with a rigid control sleeve connected to the rotor blade at the end of the flexbeam. Compact damper units are fitted between the end of the flexbeam and the rotor blade, straddling the flexbeam/blade connection and inside the control sleeve. The damping elements comprise interlaced plates connected to the rotor blade and to the damping sleeve and separated by elastomer layers.
The document U.S. Pat. No. 4,746,272 relates to helicopter rotor design and, more particularly, to flexbeams, and describes a flexbeam having a hub portion, a flap flexure portion outboard of the hub portion, a lag-torsion flexure portion outboard of the flap flexure portion, and a blade attachment portion outboard of the lag-torsion portion, and having fibers of composite material extending longitudinally the length of the flexbeam, wherein a plurality of longitudinal lobes defined by a plurality of radiused grooves extending into the lag-torsion flexure portion of the flexbeam from the upper and lower surfaces of the lag-torsion flexure portion to a central planar portion of the lag-torsion flexure portion, the grooves being radiused at their bases to enhance the torsion flexure characteristics of the lag-torsion flexure portion.
Other prior art documents were considered for drafting the application, i.e.: CA2042532, CA2320606, CA2650760, CN102167156, DE10316093, DE2917301, DE3534968, EP0155444, EP0097885, EP1431176, FR2474395, FR2041747, JP61021894, JP2010143484, U.S. Pat. No. 4,650,401, U.S. Pat. No. 4,746,272, U.S. Pat. No. 4,898,515, U.S. Pat. No. 5,358,381, US2004/0146403, US2006/0165527 and WO200489748.